Volume control system



July 20, 1937. v s, A, JR 2,087,316

VOLUME CONTROL SYSTEM Filed Dec. 11, 1955 2 Sheets-Sheet 1 IE2: LOSS DEV/CE H nIV/CE 4' 1 J 9 Lil c: 6 8

-II 5 L -/0 L035 DEVICE L- LOSS DEV/CE 1 f l I f l CONTROL 2x257; c/Rcu/T 32 O Q INVENTOR S. DOBA JR.

A TTORNEV July 20, 1937. s. BA, JR 2,087,316

' VOLUME CONTROL SYSTEM Filed Dec. 11, 1935 2 Sheets-Sheet 2 /Nl EN7"OR 5. DOBA JR.

A T TORNEY Patented July 20, 1937 PATENT OFFICE vommn: common SYSTEM Stephen Doba,

Jr., Woodside, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, .N. Y., a corporation of New York Application December 11, 1935, Serial No. 53,944 21 Claims. (Cl. 178-44) The invention relates to signal transmission systems and particularly to circuits for controlling the volume range of signals in such systems.

An object of the invention is to control the amplitude level of signals in a desired manner at one or more points in a signal transmission system.

Another and a more specific object is to reduce the energy volume range of signals to be transmitted at one point in a signal transmission system and to restore the signals to the original volume range at another point therein.

In signal transmission systems, such as telephone systems, it is often desirable to transmit faithfully signals having an extremely wide range of volumes, such as music, over transmission circuits in whichthe volume range of currents which can be satisfactorily transmitted, is limited on the one hand by interference of line noises, and the other hand by the necessity of avoiding distortion due to overloading transmission apparatus such as repeaters. In the prior art this has been accomplished by employing amplifying circuits to introduce a variable loss in the path of signals at the transmitting end of a system, which is a function of the signal power level and such as to compress the signal energy volume range within the transmission limits of the signal line, and by employing at the receiving end of the system circuits to introduce a variable loss in the signal line which effectively expands the received signals to their original volume range at the transmitting end of the line.

The copending application of R. W. Cushman and H. E. Hill, Serial No. 672,964, which was issued March 24, 1936 as Patent No. 2,035,263, filed May 26, 1933, discloses circuits of the abovedescribed type in which the impedance elements controlling the variable loss introduced by the amplifying circuits in the line comprise a material having a non-linear voltage current characteristic, such as a composition of silicon carbide crystals and an insulating binder (kaolin), commercially known as Thyrite, which is described in the patent to K. B. McEachron, No.

1,822,742, issued September 8, 1931, and are inserted in the input of the amplifiers. The impedances of these non-linear elements are controlled by direct current varying inaccordance with the signal power level on the line.

The circuits of the invention are in the nature of improvements over the circuits of the abovementioned application. They differ essentially from the latter circuits in that the amplifiers are of the feed-back type,.and the variable impedance elements having a non-linear voltagecurrent characteristic, are located in the feedback paths of the amplifiers and are controlled by high frequency alternating current instead of by direct current. These changes allow a greater reduction in noise and transients to be attained, increase the frequency range and volume range of the signals which may be accurately transmitted, and increase the speed of operation of the system.

The objects and advantages of the invention will be clear from the following detailed description thereof when read in connection with the accompanying drawings in which:

Fig. 1 is a diagrammatic view illustrating a signal transmission system employing at the transmitting end a volume range reducer and at the receiving end a volume range restorer embodying the invention;

Fig. 2 is a circuit diagram illustrating in more detail one embodiment of the volume range reducer of the invention which may be used in the system of Fig. 1;

Fig. 3 is a circuit diagram illustrating in more detail one embodiment of the volume range restorer of the invention which may be used in the system of Fig. 1; and

Fig. 4 is a circuit diagram illustrating a shunt type feed-back amplifier which may be substituted for the series type feed-back amplifier in the circuits of the invention shown in Figs. 2 and 3.

In the system of Fig. 1, a signal transmission line I of limited volume range capacity, such as a telephone line, has connected to it at a transmitting point a volume range reducer or compressor 2, and at a receiving point a volume range restorer or expander 3.

The volume range reducer 2 at the transmitting end of the line I includes a loss device 4, such as a vario-amplifier connected in the line I and a similar loss device 5 in a branch circuit 6 connected to the line I at a point in the output of the loss device 4. The loss devices 4 and 5 in line I and branch circuit 6, respectively, are governed by the control circuit I which in turn is controlled from the output of the loss device 5 in such manner as to make the volume input to the branch circuit 6 substantially constant and to reduce the volume range on the transmission line I in the output of the range reducer 2 to one-half the signal volume range in the input of the reducer 2.

The volume range restorer 3 at the receiving end of the line I, comprises the loss device 8 and an amplifier 9 connected in the transmission line I, a loss device I0, similar to the loss devices 4 and 5 in the reducer at the transmitting end of the system, in a branch circuit II connected to the transmission line I in front of and adjacent the loss device 8, and a control circuit I2 also connected to the line I through the loss device I0 and the branch circuit II. The loss devices 8 and III are governed by the control circuit I2 in such manner as to maintain constant volume in the branch circuit H in the output of the loss device l0, and to restore the signals in the transmission line I in the output of the restorer 3 to the original volume range they had at the input to the reducer 2 at the transmitting end of the line I.

In Fig. 2 of the drawings is shown the circuit details of the range reducer 2 in the system of Fig. 1 in accordance with one embodiment of the invention.

As indicated in Fig. 2, the loss device 4 comprises a vario-amplifier consisting of three amplifying vacuum tubes l3, l4, and I5 coupled in tandem by resistance-capacity couplings, and a series feed-back circuit between tubes l5 and I3. A small (GOO-ohm) resistance His common to the control grid-cathode circuit and cathodeanode circuit of tube l5, and a small (GOO-ohm) resistance I8 is common to the control gridcathode circuit and cathode-anode circuit of tube l3. The control grid-cathode circuit of tube I3 also includes in series between resistance l8 and the secondary winding of the input transformer 2| another resistance l9 having the same value as resistance l8. The series feed-back path for the vario-amplifier is between the resistance I1 and the resistance l6. A bridge circuit l6 having in each of its four arms an element T of a material having a non-linear current-voltage characteristic, such as is disclosed in the aforementioned McEachron patent, is connected in the series feed-back path by connections from the respective cathodes of the tubes I5 and I3 to opposite vertices of the horizontal diagonal of the bridge circuit. The amplifying tubes l3 and I4 are screen grid tubes and the tube I5 is a triode.

The input of line I is connected to the first tube l3 of the feed-back vario-amplifier by transformer 2|, and the output circuit of the last tube l5 in the feed-back amplifier is coupled to the outgoing portion of line I by output transformer 22. The shunt resistance 23 serves as a termination of the transformer 22 since the tube does not do so because of the series type of feed-back employed.

A network 26 comprising a capacity shunted by a plurality of circuits each including a resistance and a capacity in series is connected between the cathode of tube I 5 and the grid of tube l3 through the secondary winding of transformer 2|. This is for the purpose of feeding back into the input of tube l3 a current which is 180 degrees out of phase with the current fed back through the bridge circuit l6, so as to balance or equalize for the effects of a similar network inherently a part of the elements T in the bridge circuit I 6.

Two equal large (10,000-ohm) resistances and 24 are connected in series across resistances l8 and IS in series in the cathode-control grid circuit of tube l3. The junction point of the resistances 25 and 24 is connected to the positive terminal of plate battery 26 in the plate circuit of tube l5 by the conductor 21 including the variable resistance 28. This resistance arrange ment is provided for the purpose of equalizing potential differences between the two resistances I1 and I8 so as to prevent the setting up of a direct current bias across the elements T in the bridge circuit l6. The variable resistance 28 is adjusted until the voltage drop across the vertices of the horizontal diagonal of bridge circuit I6 is zero.

The loss device 5 comprises a vario-amplifier with a series feed-back circuit, substantially identical with the feed-back amplifier in the loss device 4, as indicated by the use of similar reference characteristics for identifying the various elements of the former, but followed by a prime mark. The loss device 5 is connected in tandem with the loss device 4 through the branch circuit 6 which is connected across the line i on the output side and adjacent the output transformer 22 in the feed-back amplifier in loss device 4.

The control circuit 1 includes the step-up transformer 29, the terminals of the primary winding of which are connected to the terminals of the secondary winding of the output transformer 22' of the feed-back amplifier in loss device 5. The secondary transformer 29 is connected to the input of the full-wave detector circuit 30 comprising the two three-electrode detector tubes 3| and 32, the

control grid-cathode circuits of which are connected in push-pull relation. The grids of tubes 3| and 32 are negatively biased by the common battery 33 so that the bias is much more than enough-to zero. Hence, the detector will act as a marginal device, that is, the plate current thereof will not start flowing until a critical value of input has been exceeded, beyond which the plate current will increase rapidly.

When the input to the detector 30 exceeds the critical operating value, plate current flows from the common plate battery 34 over a circuit which may be traced from the positive terminal of the battery through resistance 60, anode-cathode circuits of tubes 3| and 32 in parallel, and through resistances 31 and 39 in series to the negative terminal of battery 34. The plate current also appliesa positive charge to condensers 35 and 36 through resistance 31 and the twoelement rectifier tube 38, respectively. The discharge path of condenser 35 is through resistance 39 and the discharge path of condenser 36 is through the resistance 40, and the resistance 39 and condenser 35 in parallel. The condenser 36 is made much smaller than condenser 35, and resistance much larger than resistance 39 (in the same ratio).

Then the discharge of condenser 36 may be expressed approximately by the following equation: l max 1 where E; =the instantaneous voltage across condenser 36 as the condensers 35 and 36 discharge.

Emax=the maximum voltage to which condensers 35 and 36 are charged.

35 and resistance 39,

=the product of the values of condenser 36 and resistance 40.

The function of the two-element-rectifier 38 is to enable the rapid charging of condenser 36.

winding of the step-up reduce the plate current in the tube to That is, when condenser 36 is being charged, the

conduction is through the rectifier 38, and when condenser 36 is discharging, the resistance of the rectifier 38 is infinite and conduction is through resistance 40. The purpose of resistance 3'! is to insure the charging of condenser 35 at the same rate and to the same value as condenser 36. It (resistance 31) merely corresponds to the conducting resistance of rectifier 38. 3

Tube 4| is a high frequency modulator of any suitable type for supplying high frequency (say 700 kilocycles) voltage of controlled amplitude across the vertical diagonal of each of the bridge circuits |6, |6 through the high frequency transformers 42 and 43, respectively. The source of high frequency is the oscillator 44 of conventional type coupled to the input circuit of the modulator tube 4| by a transformer 45,

The condenser 36 in the control circuit 1 is connected across the control grid-cathode circuit of tube 4|, so that the charge on condensers 35 and 36 controls the bias on the modulator tube 4|. The control grid of the modulator tube 4| is negatively biased by the battery 46, this bias being made great enough so that with no charge on condenser 36, there is no transmission of high frequency to the circuits I6, I6. The parallel resistance-capacity network 41 in the control grid-cathode circuit of the modulator tube 4| is provided to take care of the condition of a positive bias on condenser 36 greater than the value of the battery 46. This would make the grid of tube 4| conductive and a low impedance, and might stop oscillator 44 from oscillating. If this happens, the whole circuit would be disabled and would not restore itself to a normal working condition. However, the presence of the network 41 insures against this by effectively limiting the current that can flow through the control grid of tube 4|. Incidentally, the modulator 4| may comprise two or more tubes in parallel.

The functions of the various elements of the volume range reducer or compressor shown in Fig. 2 will be brought out in more detail in the following complete description of operation.

Let it be supposed that itds desired to transmit over the transmission line signals having a Wider range of volumes than can be handled satisfactorily by the line for example, signal waves corresponding to a musical program.

Referring to Fig. 2, the signal waves received in the input of line l are impressed by the input transformer 2| on the input of the loss device 4 and are amplified by the amplifying tubes l3, l4, and therein. The amplified waves in the output of tube l5 are impressed by the output transformer 22 on the outgoing portion of the line Part of the amplified signal energy in the output of the loss device 4 is also picked off by the branch circuit 6, and is amplified by the variorepeater tubes; l3, l4, and I5 in loss device 5. The amplified signal energy in the output of loss device 5 is impressed by input transformer 29 on the input of the control circuit 1. If the impressed energy is sufiicient to overcome the normal negative bias on the grids of the tubes 3|, 32 in the full-wave detector 36, plate current will flow in the output circuit thereof and will cause a positive biasing charge to be applied to the condensers 35 and 36 through resistance 31 and rectifier 38, respectively. This charge will be proportional in value to the amplitude level of the signal energy applied to the control circuit 1. The condensers 35 and 36 so charged effectively decrease the negative bias on the control grid of the modulator tube 4|, causing high frequency current energy of proportional amplitude to be supplied from its output to the non-linear elements T in bridge circuits l6 and I6 through the high frequency transformers 42 and 43, respectively.

The impedance of the non-linear elements T in the bridge circuits l6, I6 is reduced in direct proportion to the amount of high frequency current flowing through them. As the amount of high frequency current increases with an increase in the positive charge on condenser 36, the amount of negative feed-back through the bridge circuits l6, l6 increases proportionately and therefore the gains of the vario-amplifiers in the loss devices 4 and 6 are simultaneously and equally reduced. This reduction in gain continues until the energy applied to the input of the detector 30 by transformer 29 is lower than the critical value necessary for the operation of the detector tubes 3| and 32. For a steady input, a point of equilibrium is soon reached.

If the level of the signal waves applied to the input of the reducer 2 and thus the level of the waves applied to detector 30 is again increased over the critical operating value of the detector, the positive charge stored in condenser 36 is increased proportionately, reducing the gain of the feed-back amplifiers in loss devices 4 and 5 until the energy level of the waves applied to the detector 36 is again reduced below a critical operating value. The increased input level of the waves in the line I applied to the volume range reducer 2, hence, results in an equal decrease in gain in the feed-back amplifiers of loss devices 4 and 5, half of which only is inserted in the line I, so that the increase in signal level in the transmission line in the output of reducer 2 will be one-half the increase in level of the original signals applied to the input of the reducer 2. If the volume level of the signals applied to the detector 36 is above the. marginal operating value and then the signal input level decreases, the

positive charge on condensers 35 and 36 will dei crease and the amount of high frequency current supplied by tube 4| to the non-linear elements T in bridge circuits |6, |6 will decrease. The resultant increase in impedance of the nonlinear elements in the bridge circuits will cause the negative feed-back in the vario-amplifiers in loss devices 4 and 5 to be proportionately decreased and the gain, therefore, of the vario-amplifiers to be increased. However, in this case the combination of resistance 31, resistance 39, condenser 35, resistance 4|], the rectifier 38 and condenser 36 in control circuit 1 willintroduce a desirable time lag between the decrease in signal input level of the reducer 2 and the resultant increase in gain of the vario-amplifiers in loss devices 4 and 5. A time lag of this sort is desirable since a too rapid variation in gain results in the generation of sum and difference frequencies that would not be part of the original signal. Another effect would be the rapid variation of noise amplitude at the expander at the other end of the line. In certain cases (with high noise) this also would be objectionable. In general it is desirable to make the reduction in gain as quick as possible to reduce overloading on peaks and the increase in gain as slow as possible for the above-mentioned reasons.

Fig. 3 shows in detail the preferred construction of the volume range reducer or expander 3 connected to the output of transmission line in the system of Fig. 1. As indicated in Fig. 3, the loss device 8 and the amplifier 9 of the restorer circuit 3 are connected in tandem in the line I. The loss device 8 includes a bridge circuit 56 comprising in each arm thereof an element T having a non-linear voltage current character istic, as in the bridge circuits I6 and I6 in the range reducer circuit of Fig. 2, and the two tandem-connected transformers 5| and 52 in the line I. The bridge circuit 50 is connected di ectly in series with the line I between the transformers 5| and 52 by connections from the lower terminal of the secondary winding of transformer 5| and the lower terminal of the primary winding of transformer 52, respectively, to opposite vertices of the horizontal diagonal of the bridge circuit.

Connected in series across the secondary winding of the transformer 5| are the two equal (small) resistances 53 and 54, which correspond respectively to the resistance I8 and the resistance I9 in loss device 4 in the circuit of Fig. 2. The primary winding of the transformer 52 is terminated by the shunting resistance 55, which resistance corresponds to the resistance I1 in the reducer circuit of Fig. 2. of the two resistances 53 and 54 is connected to ground, and to the upper terminal of the primary winding of transformer 52.

The capacitance-resistance network 56, which has a characteristic equivalent to a network inherent in the non-linear impedance elements T in bridge circuit 50, is connected in series between the upperterminal of the secondary winding of transformer 5| and the lower terminal of the primary winding of transformer 52, and thus across the bridge circuit 50, the resistance 53 and the resistance 54 in series. The network 56 as connected transmits over the line I between the transformers SI and 52 signal waves which are 180 degrees out of phase with those transmitted therebetween through the bridge circuit 50, and thus balances or equalizes for the excess of high frequencies transmitted by network 50.

The input of the loss device I is connected across the line I in front of and adjacent the loss device 8 by the branch circuit II. The loss device I 0 comprises a vario-amplifier of the series feed-back type identical with the feed-back vario-amplifiers in the loss devices 4 and 5 in the range reducer shown in Fig. 2, as indicated by the use of the same reference characters for identifying the component parts as used for the corresponding parts in Fig. 2, except that the reference characters in the case of loss device I0 are followed by a double prime mark.

be control circuit I2 connected to the output of the loss device I0 is identical with the control circuit 1 in the range reducer circuit 2 shown in Pg. 2, as indicated by the use of the same reference characters identifying the component parts as used in Fig. 2 except that in control circuit I2 the reference characters are followed by a prime trols the impedances of the non-linear elements T in the bridge circuit I6" in the feed-back path of the amplifier in loss device I0 and of the nonlinear elements T in the bridge circuit 50 in series with the line I, through the high frequency transformers 43' and 42, respectively, in accordance with changes in the signal volume range in line I in the input of the restorer circuit in the manner which will be described. I

The operation of the range restorer circuit of Fig. 3 is as follows:

The signal waves received over the transmission line I are divided between the input of loss device 8 in line and the input of branch circuit II. The portion entering loss device 8 passes through transformer 5|, the resistance network 53, 54,

The junction point mark. The control circuit I2 conbridge circuit 50 and resistance-capacity network 56 and are then impressed by transformer 52 on the input circuit of the amplifier 9 in which they are amplified to the desired degree.

The portion diverted into the branch circuit II is transmitted by transformer 2|" and are amplifled by the amplifying tubes I3", I and I5" in loss device I0. The amplified signals in the output of loss device I0 are impressed by the step-up transformer 29" on the input of the full-wave detector 30' of control circuit I2.

Whenever the volume range of the signals received over the line I is such that the energy level of the waves impressed on the detector 30' exceeds the critical operating value of tubes 3|, 32' as, in the same manner, previously explained in connection with the similar control circuit shown in Fig. 2, the positive bias on the condenser 36 is increased with a resultant increase in high frequency energy supplied through transformers 43' and 42' to the non-linear impedance elements T in the bridge circuits I6" and 50 in the series feed-back path of the vario-amplifier of loss device III and in series with the line I, respectively. The effective impedance of the bridge circuit I6" and hence the gain of the vario-amplifier in series in the feed-back path of the vario-amplifier in loss device It), is decreased. This decrease of gain continues until the level of the energy impressed on the detector 30' ing value of tubes 3|, 32' come inoperative and the supply of high frequency energy to the circuits I6" and 50 is cut off.

The impedance of the nonlinear elements T in the bridge circuit 50 is also decreased with the increase in high frequency energy applied thereto, but, because the circuit 50 is connected in series with the line I, its decreased impedance results in a decrease in loss in the line I, or an effective increase in gain. This increase in gain is equal to the decrease in gain of the vario-amplifier in loss device I0, and is equal in value to the increase in input to the range restorer 3. Hence, when the restorer 3 of Fig. 3 is used in combination with the range reducer of Fig. 2 in the system of Fig. 1, the volume range of the signals in the output of the restorer 3 at the receiving end of when these tubes bethe line I will be substantially the same as thevolume range of the signals in the input of the reducer 2 at the transmitting end of the line I.

Also, when the level of the signals impressed on the input of the range restorer of Fig. 3 is above that value which will cause the waves applied to the control circuit I2 to exceed the operating value of detector tubes 3|, 32', and then this input level decreases, the reduction in the positive charge on the condenser 36 will reduce the biason the modulator tube 4| and result in a decrease in the amount of high frequency energy applied therefrom to the bridge circuits I6" and 50 through the transformers 43 and 42, respectively. The reduction in the current then flowing through the non-linear elements T in these circuits will result in an increase in gain in the vario-amplifier of loss device I0 and an effective decrease in gain in line I provided by loss device 8. The decrease in gain produced by loss device 8 is equal to the increase in gain of the vario-amplifier in loss device I II, and equal in value to the decrease in the input level to the range restorer.

Fig. 4 shows a vario-repeater with a shunt feedback circuit which may be substituted for the series type feed-back vario-amplifier in the loss devices 4 and 5 in the reducer circuit of Fig. 2

falls below the operatand in the loss device III in the restorer circuit of Fig. 3.

The vario-amplifier of Fig. 4 comprises the three resistance-capacity coupled amplifier tubes cuit comprising condenser 68, resistance 69 and resistance 10 in series. Condenser 68 is a blocking condenser and resistances 69 and 10 comprise a potentiometer.

The feed-back path of the amplifier is between resistance 10 and resistance 65, and includes in series the bridge circuit ll comprising in each of the four arms thereof an element T having a non-linear voltage current characteristic, similar to the bridge circuits I6, I6 in the circuit of Fig. 2 and the bridge circuits 50 and 16" in the circuit of Fig. 3. The secondary winding of the high frequency transformer 12 is connected across the vertical diagonal of the bridge circuit- H, and the value of the impedance of the nonlinear elements T in the bridge circuit H is controlled by the variable-amplitude alternating current supplied across the terminals of the 'primary winding of that transformer from an associated control circuit (not shown) similar to and operating in similar manner to the control circuits I and I2 in Fig. 2 and in Fig. 3, respectively.

To equalize for the non-linear frequency response of the impedance elements in bridge circuit H, use is made of an element 13 having a non-linear frequency characteristic, similar to that of the elements T in the bridge circuit. The element 13 is connected in series with the condenser M in a connection from a point between resistances B9 and 10 in the output circuit of tube 63 and a variable tap on the resistance 64 in the input circuit of tube iii. The proper adjustment to equalize for the non-linear frequency response of the bridge circuit H is made by properly adjusting the potentiometer 64. To insure that the transmission between the resistances l0 and 65 through bridge circuit H is greater at all times than between resistance 10 and potentiometer resistance 64 through the element 13,- the bridge circuit II is shunted by the resistance 15, The resistance 18 in the plate circuit of tube 63 is used to provide the terminating resistance for the output transformer 61, since the impedance of the amplifier tends to be a short circuit due to the shunt feed-back.

The vario-amplifier circuit of Fig. 4 is simpler than that of the vario-amplifier circuits in loss devices 4, 5, and i0 shown in Figs. 2 and 3, respectively, due to the elimination of the resistance network means for obtaining a direct current balance in the latter circuits.

The use of the feed-back amplifiers in the volume range reducer and restorer circuits of the invention as described above has an advantage over the similar circuits of the prior art employing ordinary amplifiers from the standpoint of reduction of noise and transients in that the gain of the amplifiers is not constant, but is decreased at the time the transients are introduced in the circuit by the variation in the impedance of the controlling element. The use of the impedance elements having a non-linear voltage characteristicin the feed-back paths of the amplifier has the further advantage that the gain of the amplifier is reduced as the impedance of the non-linear elements is decreased and would be zero at the time the greatest transient is introduced.

A further advantage from the standpoint of the reduction in noise and transients is obtained by the use of a high frequency current instead of direct current as in the prior art circuits to control the impedance of the non-linear elements. If the non-linear elements used are substantially symmetrical in characteristic as would be the case where the material therein is such as disclosed by the McEachron patent referred to, little or no rectification would result from the application of a high frequency control current, and, if the high frequency itself were not passed by the amplifier, there would be no resultin transients.

The noise from the non-linear elements is reduced in much the same manner as are the transients. This noise is a function of current through the non-linear elements, being greatest when the current is at a maximum. However, the gain of the amplifier is at a minimum at this time, and hence the noise output actually decreases as the gain decreases.

Various modifications in the circuits illustrated and described which are within the spirit and scope of the invention will occur to persons skilled in the art. The invention is only to be limited in accordance with the appended claims.

What is claimed is:

1. In combination in a signal wave transmission system, a transmission line, a loss network connected effectively in said line, said network including an element having a non-linear voltage current characteristic, a source of alternating voltage, and means responsive to changes in the amplitude level of the signals in said line to apply to said element a proportional alternating voltage from said source to control the transmission loss to transmitted signals introduced by said network in said line.

2. The system of claim 1, in which said control voltage is a high frequency voltage.

3. In combination in a wave transmission system, a transmission line, a wave translating device having an input circuit and an output circuit, connected in said line, a feed-back path connecting said output circuit to said input circuit, an impedance element having a non-linear voltage current characteristic, in said feed-back path, the value of said impedance element determining the amount of feed-back, and means to apply to said impedance element a control voltage to vary its impedance and thus to control the gain of said translating device. 7

4. In combination in a wave transmission system, a transmission line, a wave amplifier connected therein, a feed-back circuit connecting the output of said amplifier to its input, said feed-back circuit including an impedance element having a non-linear voltage current characteristic, the value of said impedance element con.- trolling the amount of feed-back and thus the gain of said amplifier in said line, and means to apply to said impedance element a control voltage to control the value of the impedance of said impedance element.

5. In combination in a wave transmission system, a wave amplifier having input and output circuits associated with said line, a feed-back circuit connecting said output and input circuits, a variable impedance device in series in said feedback circuit, the value of the impedance of said device determining the amount of current feedback and thus the gain of said amplifier, said variable impedance device consisting of impedance elements having a non-linear voltage current characteristic, and control means to apply across said elements an alternating voltage to control the gain of said amplifier.

6. In combination in a signal transmission system, a signal transmission line, an amplifier having a series feed-back circuit, connected in said line, a variable impedance network comprising a plurality of impedance elements in series with said feed-back circuit, each of said impedance elements having a non-linear voltage current characteristic, and means to apply across said impedance elements an alternating voltage the value of which varies in direct proportion to variations in the amplitude level of the signal waves, above a certain minimum level, in said line, to reduce the gain of said amplifier for increases in signal level and to increase the gain for decreases in signal level.

7. The system of claim 6, in which said impedance elements are connected in bridge'formation, one diagonal of the bridge being connected in series in said feed-back circuit, and said alternating voltage being applied across the other diagonal of said bridge.

8. In combination in a signal wave transmission system, a transmission line, a wave amplifier having an input and an output circuit, connected in said line, a feed-back circuit of the series type connecting said output circuit and said input circuit, an impedance element connected in series 'in said feed-back circuit, said impedance element having a non-linear voltage current characteristic, a control circuit connected across said line in the output of said amplifier, and responsive to impressed signal waves above a certain minimum amplitude level to apply across said impedance element a control voltage which increases with increases in the amplitude level of the applied signals and decreases with decreases in their amplitude level, and means in said control circuit for producing a desired time lag in the change in value of the control voltage applied to said impedance element for decreases in signal level.

9. The system of claim 5, in which said feedback circuit is of the shunt type, and said control means causes the alternating voltage applied across said impedance elements to increase with increase in the amplitude level of the waves in said line and to decrease with decrease in the amplitude level of said waves.

10. In combination in a wave transmission system, a transmission line, a loss network connected between two parts of said line, said network including a solid impedance element having a nonlinear voltage current characteristic, in series with said line, means responsive to waves in said line above a certain minimum amplitude level, to apply to said element a control voltage which increases with increases in the amplitude level of the waves in said line and decreases with decreases in their amplitude level, to vary the loss introduced by said network in said line, and a circuit connecting said two parts of said line and including means having an impedance characteristic similar to that inherent in said impedance element, in such manner as to transmit between said two parts of said line waves degrees out of phase with respect to the waves transmitted therebetween through said solid impedance element.

11. In combination in a wave transmission system, a transmission line, a wave amplifier having an input and an output circuit, connected in said line, a feed-back circuit connecting said output and said input circuit and including a series impedance element having a non-linear voltage current characteristic, means to apply a control voltage across said impedance element which, above a certain minimum level, varies in value in direct proportion to variations in the level of the waves in said line, a second feed-back circuit between the output and input circuit of said amplifier, 180 degrees out of phase with respect to the first feedback circuit, and including impedance elements having a characteristic such as to compensate for the non-uniform frequency characteristic of the first element.

12. A system for compressing the volume range of signal waves transmitted over a signal transmission line, comprising a wave amplifier having a. series feed-back circuit, connected at a transmitting point in said line, said feed-back circuit including in series at least one impedance element having a non-linear voltage current characteristic, a second wave amplifier substantially identical with the first amplifier, having its input connected across said line in the output of said first amplifier, a control circuit connected to the output of the second amplifier and responsive to variations in the amplitude level of the signal waves in the output thereof, if above a certain minimum level, to apply control voltages across the series non-linear impedance elements in the feed-back circuits of the two amplifiers which increase in direct proportion to the increases in signal level and decrease in direct proportion to decreases in the signal level, and means associated with said first amplifier for compensating for non-linear frequency effects introduced therein by the non-linear impedance elements in its feed-back circuit.

13. The system of claim 12 and in which the control voltages applied across the impedance elements in the feed-back paths of said amplifiers are alternating voltages.

14. The system of claim 12 and in which the last-mentioned means comprises a shunt feedback circuit including a network simulating in impedance characteristics a network inherent in the non-linear impedance element in the feed-back circuit of said first amplifier.

15. A system for expanding the volume range of signal waves received over a signal transmis sion line, comprising a loss network connected in said line at a receiving point, said loss network including an impedance element having a nonlinear voltage current characteristic, efiectively in series with said line, a wave amplifier having a series feed-back circuit, with its input connected across said line in front of and adjacent said loss network, said feed-back circuit including in series a second impedance element having a non-linear voltage current characteristic, a control circuit connected to the output of said wave amplifier and responsive to variations in the volume level of the signal currents in the output thereof, above a certain minimum level, to apply control voltages across the non-linear impedance elements in series with said line and in the feed-back circuit of said amplifier, respectively, which increase proportionately with increases in the signal level and decrease proportionately with decreases in signal level, and means for compensating for the non-uniform frequency response of the non-linear impedance element in series with said line.

16. The system of claim 15, in which said control voltages are alternating voltages.

17. The system of claim 15, in which said compensating means comprises a circuit for transmitting over said line signal currents which are 180 degrees out of phase with the signal currents transmitted thereover through the non-linear impedance element in series with said line.

18. The system of claim 11, in which said control circuit comprises a marginally operated wave rectifier, a network connected across the output of said rectifier comprising a shunt resistance followed by two shunt condensers, a two-element rectifier shunted by a second resistance connected between the two condensers, and in series with the output of the first rectifier, so that when the first rectifier operates, one, of said condensers is charged by its output current through said second resistance, and the other condenser is quickly charged through said two-element rectifier, and when the output current .of the first rectifier ceases, said one condenser discharges through the first resistance and said other condenser discharges through the two resistances and said one condenser, and means responsive to the charge on said second condenser ,for controlling the amount of control voltage which is applied acrossthe two non-linear impedance elements inthe feed-back circuits of the two amplifiers.

19. In combination, a source of control waves of variable amplitude level, a wave rectifier sup plied with said waves and operative when their amplitude level exceeds a certain minimum value,

a network connected across the output of said rectifier, said network including a shunt resistance, two shunt condensers and a two-element rectifier shunted by a second resistance connected in series between said two condensers so that when the first rectifier becomesoperative one of said condensers is charged by the voltage drop in the first resistance and the other condenser is charged up quickly throughsaid two-element rectifier, and when-the first rectifier becomes inoperative, said one condenser discharges through said first resistance, and said other condenser discharges through said second resistance as well as through said one condenser and said first resistance, and means for picking off and utilizing the charge on said other condenser.

20. In combination, in a signal wave transmission system, a transmission line over which signal waves are transmitted, a loss network connected effectively in said line, said network including a solid impedance element having a non-linear voltage current characteristic, an alternating current source connected across said element, and means responsive to changes in the amplitude level, above a given level, of the signal waves in said line, to proportionately control the amount of alternating current from said source transmitted through said impedance element, and thus to vary the transmission loss introduced by said network in said line.

21; The system of claim 20, in which said solid impedance element comprises a resistance material including a mass of silicon carbide crystals and a binder holding adjacent crystals in contact.

STEPHEN DOBA, JR. 

